The present invention relates to a method for measuring the concentration of a stable free radical compound in a fluid in a process of treating a radical polymerizable compound containing the stable free radical compound.
Radical polymerizable compounds such as ethylene, propylene, butadiene, acrylic acid, acrylates, methacrylic acid, methacrylates, acrylonitrile, methacrylonitrile, styrene, maleic acid and vinyl acetate, or mixtures containing two or more of them proceed in radical polymerization by the action of heat, peroxides, metal ions and the like to produce polymerized products, which adhere to distilling columns, reaction columns, heat exchangers such as reboilers, liquid transportation pipes, and the inner parts of apparatuses to result in serious operational obstacles such as a reduction in heat conductivity and clogging of the liquid transportation pipes. According to circumstances, it becomes necessary to interrupt the operation and remove the polymerized products, which sometimes causes consumption of huge cost and time.
For solving this problem, methods of inhibiting polymerization by addition of polymerization inhibitors have been used. As the polymerization inhibitors, there have been widely used aromatic amine compounds, hydroxylamine compounds, phenol compounds, quinone compounds and nitrous acid compounds. In recent years, however, attention has been focused on stable free radical compounds having steric hindrance groups represented by 2, 2, 6, 6-tetramethylpiperidine-1-oxyl because of their extremely high polymerization inhibition effect.
The polymerization inhibition of polymerizable unsaturated compounds by N-oxyl compounds has been disclosed in Japanese Patent 2818977 (corresponding to WO96/16921), JP-B-4-26639 (corresponding to WO87/04179), JP-A-10-1505, JP-A-10-158313, JP-A-10-182545, JP-A-10-182605, U.S. Pat. No. 5,880,230 and JP-A-11-222462 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d and the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d).
The stable free radical compounds scarcely react with each other because three-dimensionally large groups are adjacent to radicals in the stable free radical compounds, but can rapidly react with radicals from the radical polymerizable compounds to inhibit polymerization. The consumption of the radicals of the stable free radical compounds thus proceeds, so that the polymerization inhibition effect is lost in the state of insufficient concentration, resulting in the same state that no polymerization inhibitor is added. Industrially, from the viewpoint of safety operation, the stable free radical compounds are used in large amounts in many cases, and added exceeding a required amount, which raises an economical problem. A method has therefore been eagerly desired by which the concentration of the stable free radical compounds can be measured for a short period of time so that the compounds can be added depending on fluctuations in process conditions.
It has been disclosed in Bis [3-tert-butyl-5-(N-oxy-tert-butylamino)phenyl]Nitroxide in a Quartet Groud States; A Prototype for Persistent High-Spin Poly [(oxyimino)-1,3-phenylenes], T. Ishida, H. Iwamura (J. Am. Chem. Soc. 113, 4238-4241 (1991)), U.S. Pat. Nos. 5,780,257 and 5,811,305 that the stable free radical compound can be measured by means of electron spin resonance.
An object of the invention is to provide a method for measuring the concentration of a stable free radical compound quickly and with high precision.
As a result of intensive studies of methods for measuring the concentration of stable free radical compounds, the present inventors have found that electron spin resonance (hereinafter referred to as xe2x80x9cESRxe2x80x9d) method is simplest and can determine the concentration with sufficient precision, and achieved the invention.
That is to say, a gist of the invention is a method for measuring a stable free radical compound by electron spin resonance method in a process of treating a radical polymerizable compound containing the stable free radical compound, and another gist of the invention is the method as described above, wherein the stable free radical compound is a 2, 2, 6, 6-tetramethylpiperidine-1-oxyl compound.